Lithium ion secondary battery

ABSTRACT

A lithium ion secondary battery comprises a non-aqueous electrolytic solution, and a current collector which is coated with an active material and immersed in the electrolytic solution. By setting the coating thickness of said active material 5-80 μm, the power density of the lithium ion secondary battery is improved.

FIELD OF THE INVENTION

[0001] The present invention relates to a lithium ion secondary battery used in a hybrid vehicle.

BACKGROUND OF THE INVENTION

[0002] A lithium ion secondary battery disclosed by Tokkai Hei 5-29022 published in 1993 by the Japanese Patent Office, comprises positive/negative electrodes, an active material which performs a reversible electrochemical reaction with lithium ions, and a non-aqueous electrolytic solution which allows lithium ion transport.

[0003] The lithium ion secondary battery can be repeatedly charged and discharged, and since it is lightweight, it is also used as a battery of a hybrid vehicle. A hybrid vehicle is a vehicle provided with an engine and an electric motor as sources of drive force, and it can run on either or both of these sources.

SUMMARY OF THE INVENTION

[0004] In the low speed, low load region, the hybrid vehicle runs on the motor which has a higher performance than the engine in this region, and in the high speed, high load region, it runs on the engine which has a higher performance than the motor in this region.

[0005] The motor is mainly used during vehicle start, acceleration and deceleration. Therefore, the battery which supplies power to the motor must have a high power density so as to supply a large amount of power in a short time.

[0006] It is therefore an object of this invention to provide a lithium ion secondary battery of high power density.

[0007] In order to achieve the above object, this invention provides a lithium ion secondary battery comprising a non-aqueous electrolytic solution, and a current collector which is coated with an active material and immersed in the electrolytic solution. The coating thickness of the active material is set to 5-80 μm.

[0008] The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 is a cross sectional view of a lithium ion secondary battery according to this invention.

[0010]FIG. 2 is an enlarged view of a part A of FIG. 1.

[0011]FIG. 3 is a diagram showing the relation of the coating thickness of an active material and power density of the lithium ion secondary battery.

[0012]FIG. 4 is a diagram showing the relation of the particle size of the active material and power density of the lithium ion secondary battery.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Referring to FIG. 1 and FIG. 2 of the drawings, a lithium ion secondary battery comprises film-like positive electrodes 1, separators 2 and negative electrode materials 3 which are laminated on each other and immersed in a non-aqueous electrolytic solution 4.

[0014] The positive electrode 1 is formed by coating a positive electrode active material 6 on the surface of an aluminum current collector 5, and a negative electrode 3 is formed by coating a negative electrode active material 8 on the surface of a copper current collector 7.

[0015] Here, metal oxides such as LiCoO₂, LiNiO₂, LiMn₂O₄, Li_(x)FeO_(y) and Li_(x)V_(y)O_(z), compound oxides wherein part of these elements are replaced by other elements, e.g., Li_(x)Co_(y)M_(z)O₂ (M=Mn, Ni, V, etc.) or Li_(x)Mn_(y)M_(z)O₄ (M=Li, NI, Cr, Fe, Co, etc) where the metal element is replaced, or Li_(x)Mn₂O₄-aF_(b) and Li_(x)Co_(y)NI_(z)O_(w)F_(a) where oxygen is replaced by fluorine, are used for the positive electrode active material 6. Carbons such as graphite, mesophase carbon, hard carbon or low temperature burned carbon, metal oxides such as SnB_(x)P_(y)O_(z), Nb₂O₅, LiTi_(x)O_(y), LiFe_(x)N_(y) and LiMn_(x)N_(y), or nitrides, are used for the negative electrode active material 8.

[0016]FIG. 3 shows the relation of the coating thickness to power density when an active material of 1μm particle size is used. The ordinate is relative power density when the power density for a coating thickness of 100 μm is one.

[0017] When the coating thickness is 100 μm, the energy density increases but the internal resistance also increases and the interior of the active material no longer contributes to input/output of short-term large currents, therefore, the power density falls.

[0018] On the other hand, if the coating thickness of the active material is 5-80 μm as shown in FIG. 3, the power density can be enhanced to more than twice its value for a coating thickness of 100 μm, and if the coating thickness of the active material is 8-60 um, the power density can be further enhanced to about three or more times its value for a coating thickness of 100 μm.

[0019]FIG. 4 shows the relation of particle size of the active material to power density. The ordinate is relative power density when the power density for a particle size of 10μm is one. It is required that the maximum particle size of the active material does not exceed the coating thickness.

[0020] As seen from this figure, if the particle size of the active material is 5 μm, the power density is enhanced to about twice its value for a particle size of 10 μm. If the particle size is large, diffusion in the active material becomes slower and the power density declines, so to enhance the power density, the particle size should not exceed 5 μm.

[0021] Therefore, in this embodiment, the coating thickness of the active materials 6 and 8 is 5-80 μm and preferably 8-60 μm, and the particle size of the active materials 6 and 8 is smaller than 5 μm. As a result, a lithium ion secondary battery of high power density is obtained.

[0022] The entire contents of Japanese Patent Applications P10-133070 (filed May 15, 1998) are incorporated herein by reference.

[0023] Although the invention has been described above by reference to certain embodiments of the invention, the invention is not limited to the embodiments described above. The construction and materials of the battery are not limited to the construction and materials of the above-mentioned embodiment. Modifications and variations of the embodiments described above will occur to those skilled in the art, in light of the above teachings.

[0024] The scope of the invention is defined with reference to the following claims. 

What is claimed:
 1. A lithium ion secondary battery comprising: a non-aqueous electrolytic solution, and a current collector which is coated with an active material and immersed in said electrolytic solution, wherein the coating thickness of said active material is 5-80 μm.
 2. A lithium ion secondary battery as defined in claim 1 , wherein the coating thickness of said active material is 8-60 μm.
 3. A lithium ion secondary battery as defined in claim 1 , wherein the particle size of said active material is smaller than 5 μm. 